The present application claims the benefit of Korean Patent Application No. 64273/2000 filed Oct. 31, 2000, which is herein fully incorporated by reference.
1. Field of the Invention
The present invention relates to a linear gain control amplifier, and more particularly, to a wide band linear gain control amplifier for a mobile communications system.
2. Description of the Background Art
FIG. 1 is a block diagram illustrating the construction of a conventional linear gain control amplifier. As illustrated therein, the linear gain control amplifier includes: a compensating circuit unit 10 for converting a control signal CTL inputted for linear gain control into a control voltage VCTRL having an exponential form; a first amplifier 20 for gain-controlling an input signal RF_IN according to the control voltage VCTRL to output an amplification signal AMP_OUT; a surface acoustic wave filter 30 for filtering any distortions in the amplification signal AMP_OUT and any noise present therein; and a second amplifier 40 for gain-controlling an output of the filter 30 according to the control voltage VCTRL to output a compensating signal RF_OUT.
Here, as illustrated in FIG. 2, the first amplifier 20 includes: a low-pass filter unit 21 for filtering a low frequency of a control voltage VCTRL to output a DC component; a high-pass filter unit 22 for filtering a high frequency of an input signal RF_IN; a capacitor C3 and inductor L1 for matching the input signal RF_IN inputted via the high-pass filter unit 22 and an input impedance; an amplification control transistor 23 for amplifying a high-pass filtered input signal to a predetermined level and controlling the gain of an amplification signal AMP OUT by the DC component from the control voltage VCTRL; a tank circuit unit 24 for applying a predetermined DC level voltage to the amplification signal AMP_OUT and matching the amplification signal AMP_OUT and a predetermined output impedance; and a bypass circuit unit 25 for removing an AC component applied to the amplification control transistor 23. The second amplifier 40 can be constructed to be the same as the first amplifier 20, while the filter 30 can be a surface acoustic wave filter.
The operation of the conventional gain control amplifier will now be described.
First, the compensating circuit unit 10 compensates an externally inputted control signal CTL so that the control signal output characteristics are exponentially approximated for thereby outputting a predetermined control voltage VCTRL. This is to linearize the gain characteristics of each amplifier 20 and 40 for the control voltage VCTRL by making the previously gain-compensated control voltage VCTRL have exponential characteristics, since the gain characteristics of each amplifier 20 and 40 for the control voltage are logarithmic.
The first amplifier 20 gain-controls an input signal RF_IN according to a control voltage VCTRL applied from the compensating circuit unit 10, to thus output an amplification signal AMP_OUT to the filter 30. The filter 30 filters any distortions in the amplification signal AMP_OUT and any noise present therein, to thereby output a filtered signal to the second amplifier 40. Then, the second amplifier 40 gain-controls the filtered signal again for thereby outputting a compensating signal RF_OUT.
Here, with the respect to the operation of each amplifier 20 and 40, when an input signal RF_IN is applied to the first amplifier 20, the high-pass filter unit 22 filters a high frequency of the applied input signal RF_IN, and the capacitor C3 and inductor L1 matches the input impedance of the filtered input signal RF_IN, thus minimizing a loss generated during inputting of a signal.
The amplification control transistor 23 is controlled by an output RF_IN of the high-pass filter unit 22 applied to an input gate G2, for thereby amplifying the output RF_IN to a predetermined level according to a set gain and thereafter outputting an amplification signal AMP_OUT. The above amplification control transistor 23 can be a transistor device having a dual-gate structure G1, G2 as shown in FIG. 2.
At this time, the low-pass filter unit 21 receives the compensated control voltage VCTRL output from the compensating circuit unit 10 and filters a low frequency thereof, for thereby applying a DC component to a control gate G1 of the amplification control transistor 23. As such, the gain of the amplification control transistor 23 is controlled according to the voltage applied to the control gate G1 for thereby outputting the compensated amplification signal AMP_OUT to the filter 30.
Here, since the amplification signal AMP_OUT has a high output level, the tank circuit unit 24 is added to an output terminal in order to use the source voltage VCC as a bias voltage. The tank circuit unit 24 employs a resistor R4, a capacitor C5, and an inductor L2 having particular values so that the first amplifier 20 has a desired output impedance in order to match an impedance of the filter 30.
In addition, the bypass circuit unit 25 serves to remove an AC component applied to the amplification control transistor 23. Input and output impedance are controlled to a predetermined level by making each capacitor C3, C4, and C5 to have variable characteristics.
As described above, the conventional linear gain control amplifier is allowed to have a wide gain variable range by using the first and second amplifiers 20 and 40, and allowed to have the linearity of the first and second amplifiers 20 and 40 with gain characteristics for a control voltage by exponentially compensating a control signal CTL by the gain compensating unit 10.
However, since the above-described conventional linear control amplifier includes a surface acoustic wave filter (filter 30), as well as an inductor and variable capacitors, it is difficult to make the conventional linear control amplifier into an integrated circuit (IC). Also, the necessary inductor cannot be easily formed by standard CMOS processes. Furthermore, an additional process is required to implement the necessary variable capacitors. These drawbacks result in undesirably increased fabrication costs incurred in semiconductor chip manufacturing.
Accordingly, the present invention provides a linear gain control amplifier, which can be easily implemented and formed by standard CMOS processes and can be made into an integrated circuit (IC).
The linear gain control amplifier according to an embodiment of the present invention includes a compensating circuit unit for generating a first compensation control voltage having an exponential relation to a received basic control signal, and also for generating a second control voltage associated with the first control voltage; a variable gain amplification unit for varying a gain by the compensated control voltage; and a driving amplification unit for amplifying a signal inputted from the variable gain amplification unit to an output signal of a desired size.
Advantages of the present invention will become more apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.